The present invention relates to the field of electric batteries designed for use with electric motors which are rechargeable using regenerative charging, such as batteries for electric bicycles. More specifically, the present invention relates to a reconfigurable battery, reconfigurable electric motors for use with such a reconfigurable battery, methods for reconfiguring a battery for driving variable electrical loads, and methods for reconfiguring a battery for charging and for reconfiguring electric motors for charging a battery.
The present invention is described in connection with electric bicycles where a rechargeable battery drives an electric motor. In prior art electric bicycles, in some instances the current from the battery is regulated by a speed controller that controls the motor which provides assistance to the rider. In other instances, where the rider wants to slow down or brake going downhill, the motor acts as a generator and supplies the current back to the battery, thereby achieving regenerative braking that recovers part of the energy that would otherwise be lost when using a mechanical brake alone.
An electric motor typically uses a set of magnets, for example, electro magnets and permanent magnets. As the motor turns, the attractive and repulsive forces of these magnets are regulated electrically such that the motor turns continuously in the desired direction. This could be done by electro-mechanical switches (e.g. commutators), or could be done by solid state switches (e.g. FETs—Field Effects Transistors). FIG. 1 shows an example of a motor 12 connected to a battery 10. As the current Im flows into the motor 12 and the motor turns, the motor generates a back EMF (Electro Motive Force) which is a voltage roughly proportional to the speed of the motor 12. The current Im is defined as (VB−VM)/(RM+RB) where RM is the internal resistance of the motor 12 and RB is the internal resistance of the battery. Given a fixed applied voltage VB (e.g. from the battery 10) the back EMF reduces the amount of current that flows into the motor 12, because the current flow is proportional to the difference between the motor voltage VM (back EMF) and the battery voltage VB. For example, if the motor 12 is turning (with some outside assistance) at a rate such that the back EMF equals the battery voltage VB, than there will be no current flow. If the motor 12 turns faster than this such that the back EMF is higher than the battery voltage VB, then the current flows the other way, thereby recharging the battery 10. One extreme case is a stall, when the motor 12 is at rest. In such a case, the back EMF is zero since the motor is at rest, the current flow from the battery 10 will be at its maximum, and the motor 12 will produce its highest torque.
When the bicycle is moving and the motor 12 produces a finite back EMF, the motor 12 can be used as a generator to recharge the battery 10, while achieving a desired level of braking. In order to achieve this, the voltage out of the motor 12 is increased to a level higher than the battery 10 using a device known as an inverter.
A block diagram of a typical prior art electric bicycle system without regenerative braking is shown in FIG. 2. A battery 10 provides current to a motor 12 though a speed controller 11. The speed controller 11 governs the current flow to the motor 12, thereby controlling its speed. The speed controller 11 may be set to a desired speed by a rider using a control knob 13.
A block diagram of a further prior art electric bicycle system that provides regenerative braking is shown in FIG. 3. FIG. 3 is similar to FIG. 2 but also includes an inverter 14 in parallel with the controller 11. A switch 15 is provided for coupling the motor 12 to the controller 11 (in a drive mode) or the inverter 14 (in a braking mode). During the braking mode, current is generated by the motor 12 and passed to the battery 10 by the inverter 14, in order to charge the battery.
It should be noted that a practical system involves two distinct operations, one that drives the motor and the bicycle wheel(s) by supplying current from the battery to the motor(s), and another that uses the current from the motor(s) to charge the battery to achieve regenerative braking, thereby slowing down the bicycle. It should be further apparent from FIG. 3 that in order to recharge the battery, one needs an inverter that increases the voltage from the motor to a value higher than the battery voltage, in order for the current to flow back into the battery.
For a typical rechargeable battery, the charging voltage must be higher than the battery voltage. The higher the charging voltage relative to the battery voltage, the more current flows into the battery. Controlling the charging voltage is one of the ways to control the rate of recharging, as well as the rate of braking. Another way to control the recharging rate is pulse width modulation (PWM), where a switch between the charging source and the battery regulates an on-off duty cycle. Of course, the charging voltage still needs to be higher than the battery voltage for such a device to work.
In most electric vehicles such as electric bicycles and electric cars that utilize regenerative braking, the electrical system typically consists of several subsystems, namely a motor, a speed controller, an inverter, and a battery. Sometimes the speed controller regulates both the drive and braking current via PWM. Potentially, a clever inverter design could regulate both driving and braking by regulating the voltage to the motor for driving, and regulating the voltage to the battery for regenerative braking, thereby eliminating the need for a separate speed controller.
However, an inverter is not an easy device to design or cheaply produce, as it must handle a large amount of current (especially during quick braking) and sometimes a high output voltage, while its input voltage can fluctuate over a wide range. The input voltage in this case is the back EMF from the motor, typically close to zero when the bicycle is coming to a stop, and close to the maximum battery voltage when the bicycle is coasting on a level ground at its maximum speed (usually the battery voltage limits the top speed).
Also an inverter typically achieves its functionality using rapid switching devices. One inverter design could turn the DC current from the motor to AC current first, increase the voltage using a step-up transformer, and convert the AC current back to DC in order to recharge the battery. Another inverter design could use temporary energy storage elements such as capacitors and inductors in a charge-pump configuration in order to raise the voltage. The switching frequency involved is typically in the order of 1-100 KHz. In most of the known inverter designs, the energy loss is significant, and the cost is very high due to the high current requirement (100 Amps or more) in addition to the weight. For this reason, only a small percentage of electric bicycle products incorporate regenerative braking in their design.
It would be advantageous to provide a battery and/or electric motor configuration that provides driving and regenerative braking, for example in an electric bicycle, over a reasonable range of operations without the need for an inverter.
It would also be useful to provide a reconfigurable battery and battery control system that provides duty cycle modulation of an array of battery cells for intermediate output voltage control without incurring large switching losses, while simultaneously reducing switching induced transient signals.
It would also be beneficial to provide a reconfigurable battery and battery control system that provides multiple ports for simultaneous charging and discharging of an array of battery cells. A reconfigurable battery that provides various electrical waveform patterns, frequencies, and phases would also be useful in electrical and electronics applications such as Uninterruptible Power Supplies (UPS) and in combustion engine applications such as electrical motor powered forced air induction systems.
Forced-air induction systems increase combustion engine performance by increasing intake air pressure, allowing a greater quantity of fuel to be mixed or injected into the engine with the increased air. This increased fuel and air results in increased combustion energy and increased engine power. Increased air pressure is referred to as “boost”. Forced air induction is also useful while operating combustion engines in vehicles and aircraft at higher altitudes to compensate for reduced air pressure at high altitude. Forced-air induction systems such as superchargers and turbochargers contain fans or air compressors that function to increase pressure. Drive power for supercharger systems can be mechanical, e.g. combustion engine crankshaft driven, exhaust gas driven, e.g. a turbocharger, or electrically driven, e.g. a compressor system consisting of an electric motor combined with an axial-flow or radial-flow fan.
Advantages of the Electrical Supercharger (ESC) include lack of parasitic power drain on the engine during application of boost. This is because ESC drive power comes from stored energy within the battery, or banks of batteries, and does not immediately harness engine output during acceleration as does a mechanical supercharger or turbocharger. Also, an ESC provides instant boost independent of engine RPM, because the electric motor provides its highest motor acceleration (current) at low electric motor RPM. Exhaust driven turbochargers experience a delay while the turbocharger waits for engine RPM and exhaust gas pressure to increase sufficiently to yield adequate boost (“turbo lag”). ESC's do not experience this delay. Exhaust gas driven turbochargers use a waste gate mechanism to siphon off unusable exhaust pressure. This device is not used by an ESC since only needed power is applied to the ESC during operation, saving weight, cost and design complexity. Also, for good drivability, high boost at low to mid RPM is desirable, and since boost from an ESC is independent of engine RPM, boost can be provided at the low to mid engine RPM range. An ESC produces enhanced engine torque at low engine RPM that leads to low average engine RPM for the same drivability as higher revving engines, thereby providing better gas mileage.
Boost requirements for significant engine performance gains depend on various engine characteristics including displacement, and compression ratio, but generally about 5 pounds per square inch (psi) of boost is needed. Although mechanically driven superchargers extract engine power (typically 10%-20%) while providing boost, the automotive industry has favored both mechanical and exhaust driven systems over electrical systems because the energy requirement to drive a supercharger exceeds typically sized vehicle alternators and lead-acid batteries. Electrical superchargers that use high capacity battery banks have been designed for experimental and racing applications but these battery banks need recharging prior to each use. Other forced-air induction designs using stock vehicle batteries and alternators are marketed however they have not been powerful enough for any meaningful improvement in engine performance because of insufficient electrical power to drive the motor/blower.
A battery design is needed that will supply the power requirement to drive an ESC and overcome the limitations of a typically sized automotive alternator to supply peak power requirements. Also needed is a design that will allow efficient battery charging using typical automobile alternators that supply voltages lower than the voltage needed to run an ESC. The multi-port reconfigurable battery of the present invention provides such a design.
The methods and apparatus of the present invention provide a series connected reconfigurable battery having these and other advantages.